If you do a search for Cobra spark table or GT spark table you will find that the GT has more advance at points and that is the chronic GT ping culprit. The Cobra table is an improvement, and I'm glad to see that you have the 24lb/hr injectors, otherwise, just loading that J4J1 tune would have probably been too lean.
I know the J4J1 file is a bit softer. I did loose a bit of pull at higher loads with the original J4J1 file. I did a little experiment and put in the GT spark table in the J4J1 file. Power is back but pinging is not.
I read also that the fuel tables are different on the J4J1? My learing have not brough me in the fuel stuff yet! It has to be more than the spark tables though?